This invention relates to a magnetoresistance effect film useful in a magnetoresistance effect device for reading as a signal a magnetic field intensity in a magnetic medium and, in particular, to a magnetoresistance effect film which exhibits a large rate of change in resistance in response to a small external magnetic field.
This invention also relates to a magnetic sensor for reading an information signal recorded on a magnetic medium.
In recent years, improvement in sensitivity of magnetic sensors and increase in density of magnetic recording are advancing. Concomitantly, development of magnetoresistance effect magnetic sensors (hereinafter abbreviated to MR sensors) and magnetoresistance effect magnetic heads (hereinafter abbreviated to MR heads) is making rapid progress. Each of the MR sensors and the MR heads is operable to read an external magnetic field signal as an electric resistance variation in a sensor in response to the external magnetic field. In these MR sensors and MR heads, reproduction outputs do not depend upon relative speeds with respect to recording media. This leads to high sensitivity of the MR sensors and high output levels of the MR head in high-density magnetic recording.
Recently, proposal is made of a magnetoresistance effect film which comprises at least two ferromagnetic layers or thin films stacked one over the other with a nonmagnetic layer or thin film interposed therebetween, and an antiferromagnetic layer or thin film underlying a first one of the ferromagnetic thin films so that the first ferromagnetic thin film is provided with antimagnetic force, that is, constrained by exchange anisotropy or exchange biasing.
A so-called soft magnetic material or a high permeability magnetic material is usually used for the ferromagnetic layers. The term "nonmagnetic" is usually used to mean "paramagnetic" and/or "diamagnetic".
When an external magnetic field is applied to the magnetoresistance effect film, the direction of magnetization of the other second one of the ferromagnetic thin films is rotated with respect to that of the first ferromagnetic film. Thus, change in resistance takes place. The magnetoresistance effect film of the type described is disclosed in, for example, Physical Review B, Vol. 43, pp. 1297-1300, 1991 and Japanese Patent Unexamined Publication No. 358310/1992.
Disclosure is also made of a conventional magnetic read transducer, called an MR sensor or an MR head, which can read data from a magnetic surface with high linear density. The MR sensor detects a magnetic field signal through change in resistance as a function of the intensity and the direction of magnetic flux detected by a reading element. The above-mentioned conventional MR sensor is operated on the basis of an anisotropic magnetoresistance (AMR) effect. Specifically, one component of the resistance of the reading element changes in proportion to the square of the cosine of the angle between the magnetization direction and the direction of the sense current flowing through the element. The AMR effect is described in detail in an article written by D. A. Thompson et al and entitled "Thin Film Magnetoresistors in Memory, Storage, and Related Applications", IEEE Transactions on Magnetics, Vol. MAG-11, No. 4, pp. 1039-1049, July 1975.
More recently, disclosure is made of a further remarkable magnetoresistance effect. Specifically, change in resistance of a stacked-type magnetic sensor results from spin-dependent transmission of conduction electrons between ferromagnetic layers with a nonmagnetic layer interposed therebetween and from interfacial spin-dependent scattering accompanying the spin-dependent transmission. Such magnetoresistance effect is called by various names such as "a giant magnetoresistance effect" and "a spin-valve effect". Such magnetoresistance effect sensor made of an appropriate material has improved sensitivity and exhibits large rate of change in resistance. In the MR sensor of the type described, in-plane resistance between a pair of the ferromagnetic layers separated by the nonmagnetic layer changes in proportion to the cosine of the angle between magnetization directions in the two ferromagnetic layers.
Japanese Unexamined Patent Publication No. 61572/1990 claiming a priority of June 1986 discloses a stacked magnetic structure giving large rate of change in magnetoresistance resulting from antiparallel alignment of magnetization directions in ferromagnetic layers. In the above-mentioned publication, ferromagnetic transition metals and alloys are recited as a material which can be used in the stacked structure. The publication discloses the structure including an antiferromagnetic layer attached to one of at least two ferromagnetic layers separated by an intermediate layer and mentions that FeMn is suitable as the antiferromagnetic layer.
Japanese Unexamined Patent Publication No. 358310/1992 with a priority claim of Dec. 11, 1990 discloses an MR sensor comprising two ferromagnetic thin film layers separated by a metallic nonmagnetic thin film layer. When an applied magnetic field is zero, magnetization directions of the two ferromagnetic thin film layers are orthogonal to each other. Resistance between the two uncoupled ferromagnetic layers changes in proportion to the cosine of the angle between the magnetization directions in the two ferromagnetic layers, regardless of the direction of electric current flowing through the sensor.
Japanese Unexamined Patent Publication No. 127864/1996 discloses a magnetoresistance effect device comprising a plurality of ferromagnetic thin films stacked one over the other on a substrate with a nonmagnetic layer interposed there-between, and an anti-erromagnetic thin film arranged adjacent to one of the ferromagnetic thin films. A biasing magnetic field Hr of the antiferromagnetic thin film and a coercive force Hc.sub.2 of the other magnetic thin film satisfies Hc.sub.2 &lt;Hr. The antiferromagnetic thin film comprises a superlattice composed of at least two materials selected from NiO, Ni.sub.x Co.sub.1-x O, and CoO.
Although the magnetoresistance effect device in the above-mentioned Japanese Publication No. 127864/1996 is operable in response to a small external magnetic field, a signal magnetic field must be applied in a direction of an easy magnetization axis when it is used as a practical sensor or magnetic head. When it is used as a sensor, no change in resistance is observed around the zero magnetic field and nonlinearity such as Barkhausen jumps appears.
In addition, ferromagnetic interaction is caused between the ferromagnetic thin films which are located adjacent to each other with the nonmagnetic layer interposed therebetween. As a consequence, a linear zone in an M-R curve is shifted from the zero magnetic field.
The antiferromagnetic thin film is made of FeMn which has a poor corrosion resistance. For practical use, a countermeasure against corrosion is required such as incorporation of an additive or application of a protective film.
Basically, change in resistance is obtained by the use of change in the length of mean free path of conduction electrons across the three layers of the ferromagnetic/nonmagnetic/ferromagnetic thin films. With this structure, change in resistance is small as compared with magnetoresistance effect films having a multilayer structure called a coupling type.
On the other hand, when a Ni oxide film having an excellent corrosion resistance is used as the antiferromagnetic thin film, the biasing magnetic field is small so that the coercive force of the adjacent ferromagnetic thin film is large. This makes it difficult to obtain antiparallel alignment of magnetization directions in the ferromagnetic thin films.
When the oxide film is used as the antiferromagnetic thin film, the adjacent ferromagnetic film is oxidized during heat treatment. This reduces the magnitude of the biasing magnetic field and change in resistance of the magnetoresistance effect film.
When the superlattice composed of NiO and CoO is used as the antiferromagnetic thin film, a large exchange-coupling magnetic field is obtained. However, a blocking temperature which is an upper limit of operation of the magnetoresistance effect film is lowered. In addition, production cost inevitably becomes high.
When the two-layer film composed of the NiO layer and the CoO layer deposited on the NiO layer to a thickness between 10 and 40 angstroms is used as the antiferromagnetic film, it is extremely difficult to obtain an antiferromagnetic phase of CoO.
Basically, change in resistance is obtained by the use of change in the length of mean free path of conduction electrons across the three layers of the ferromagnetic/nonmagnetic/ferromagnetic thin films. With this structure, change in resistance is small as compared with magnetoresistance effect film having a multilayer structure called a coupling type.
In the above-mentioned prior art, FeMn, which is easily oxidized in an atmosphere, is mainly recited as a candidate of the antiferromagnetic layer forming a spin-valve structure. For practical use, it is essential to take a countermeasure against the corrosion such as incorporation of an additive or application of a protective film. Even if such countermeasure is taken, deterioration in characteristics is still inevitable during manufacturing process. Under the circumstances, a manufactured device is often insufficient in reliability.
When the Ni oxide film or the CoPt film excellent in corrosion resistance is used to increase the intensity of a reverse magnetic field in one of the ferromagnetic thin films, the sandwich structure of the ferromagnetic/nonmagnetic/ferromagnetic thin films has a poor crystallinity. This results in frequent occurrence of the hysteresis on an R-H loop.
When the magnetoresistance effect device based on an SV (spin-valve) effect is used as the magnetoresistance effect sensor, optimization is required at an operation point (cross point) in the zero magnetic field, like the conventional magnetoresistance effect sensor using the AMR effect. In the magnetoresistance effect device using the SV effect, the reproduction output at the head is affected by the configuration of the device. In the SV device using a nonconductive material as the antiferromagnetic layer, the thickness of the antiferromagnetic layer affects the gap length of the shielded type magnetoresistance effect device and the off-track characteristic of the reproduction signal reproduced by the shielded-type magnetoresistance effect head.